Hitherto, white light emitting devices constituted by the combination of a gallium nitride (GaN)-based blue light emitting diode as a semiconductor light emitting element and a phosphor as a wavelength conversion material have been noticed as a light emission source for displays or lighting systems because of a less consumption of electric power and a long service life thereof.
In the white light emitting devices, the phosphor used therein absorbs a blue-range visible light emitted from the GaN-based blue light emitting diode, and emits a yellow light, so that a blue light unabsorbed by the phosphor is mixed with the yellow light, resulting in emission of the white light.
As known in the art, the above phosphor typically includes a host material composed of an yttrium and aluminum composite oxide (Y3Al5O12), and cerium (Ce) incorporated as a luminescent center ion therein. The phosphor is not necessarily easily produced owing to its high calcination temperature, etc. In addition, the phosphor tends to be deteriorated in brightness with the increase in temperature upon use. Therefore, when the phosphor is applied to displays or lighting systems, the brightness thereof tends to be lowered with the temperature rise of these apparatuses, resulting in problems such as low color rendering property of the light emitting device. Further, in the case of white LED as a typical light emitting device which is constituted by the combination of a blue LED and a yellow phosphor, the amount of light emitted therefrom in red range (not less than 600 nm) and bluish green range (480 to 510 nm) is small, and the light emitted from the white LED has a low general color rendering index Ra as well as a low special color rendering index R5 that represents a color rendering property in the bluish green range.
On the other hand, in “Optical Materials”, 6 (1996), pp. 153–159, there is described a light emitting substance composed of Cr-activated Ca3Sc2Ge3O12. However, the use of such a light emitting substance is industrially disadvantageous since a Ge compound as a raw material thereof is very expensive.
The present invention has been made in view of the above problems. An object of the present invention is to provide a phosphor capable of not only being readily produced but also providing a light emitting device having a high color rendering property, a light emitting device using the phosphor, and a display and a lighting system using the light emitting device as a light source thereof.